Plans for "affordable" town house and condominium developments in eastern Loudoun County demonstrate how much the area has changed from a rural preserve where housing was a bargain to just another expensive Washington suburb.

The Board of Supervisors approved the county's first housing aimed specifically at middle-income families this week. Peace Plantation, a large housing and office project in Sterling in eastern Loudoun, will include 73 low-cost town houses and condominiums. Two other developments including affordable units also are on the drawing board.

In pressing for more affordable housing, Loudoun joins a growing number of areas, including Fairfax and Montgomery counties, in trying to promote its development. In 1980, the average price of a single-family home in Loudoun was about $75,000. By last year, the average had risen to about $138,000.

Jinni Benson, chief of the county's Housing Services Department, said it is crucial to build new affordable housing in areas of the county where new $250,000 houses and $150,000 condominiums far outstrip most paychecks. "We want teachers and firefighters and doctors and lawyers -- everyone -- to continue to live in our community," she said. "Now we're becoming an economic force in Northern Virginia, and we also need to be a balanced community."

Benson said a person or family earning about $27,000 a year should be able to buy affordable housing. That means a town house that costs about $68,000, she said.

She estimates the county needs at least 5,000 units of affordable housing for people who already live in Loudoun. The county's Affordable Housing Advisory Committee wants 2,250 affordable units built by 1995.

The affordable town houses and condominiums at Peace Plantation will sell for under $70,000. They will be neighbors with 660 higher-priced homes and some office buildings on a 75-acre parcel near Routes 625 and 28.

Two other proposed developments in Loudoun, Ashbrook Green and Autumn Hills, would include more than 200 units of affordable housing among 800 total units. Both are expected to be reviewed by the board early next year, she said.

Those proposals were made after the county offered to allow more housing than usual on each acre of land. Such an arrangement would allow developers to sell some units at lower prices.

Loudoun's Housing Services Department intends to follow the lead of other local governments by proposing an ordinance in January that would require developers to include some low-cost housing in each project.

Benson said some areas require developers of more than 50 units to set aside about 12 percent of the units for affordable housing. In exchange, the developer would be allowed to build more housing in a given area, she said.

Charles A. Bos (D-Leesburg), vice chairman of Loudoun's Board of Supervisors, said that unless the amount of low-cost housing is increased, firms thinking about moving to Loudoun will look elsewhere.

United Airlines, for example, is considering whether its employees can afford to live in the county if the aviation giant decides to move a large maintenance facility there, he said.

"The labor pool is not going to be there without the housing," he said. "It's just critical for the future, for the overall social and economic good of the county."